1. Field of the Invention
The present invention relates to a lubricating composition.
2. Description of the Related Art
The present conditions sorrounding automobiles consist of tighter and tighter fuel efficiency standards, auto-emission standards, etc. This has arisen because of concerns about global warming, air pollution, acid rain and other environmental issues, and resource conservation out of fears about depletion of limited petroleum energy. The most effective solution to those problems at present is improvement in fuel efficiency.
Key elements for improving the fuel efficiency of automobiles are improvements in engine oils such as viscosity reduction and addition of superior friction modifiers to prevent frictional losses, and improvements in automobiles themselves such as weight reduction and engine improvements. Engine oils serve as lubricants between pistons and liners, in which area fluid friction predominantly occurs, and viscosity reduction in engine oils can reduce frictional losses. Accordingly, the viscosity of engine oils has been further reduced in recent years, but this invites new problems such as reduced sealing properties and increased abrasion loss. Engine oils also play important roles in lubricating moving valve systems and bearings, in which area mixed type lubrication and boundary lubrication predominantly occur, and thus the reduction in viscosity of engine oils causes friction increases. Therefore, friction modifiers, extreme pressure additives and others are added to engine oils in order to reduce frictional losses and to avoid abrasion arising from reduced viscosity.
Organo-molybdenum compounds have superior friction reduction activity and are added to a variety of lubricating oils. They are particularly effective in engine oils for improving fuel efficiency, to reduce frictional resistance on individual parts of engines and thereby to save fuel, and have become essential additives for so-called fuel efficient oils. Such fuel efficient oils should not only exhibit superior fuel-saving performance at early stages of their use but also retain that performance for a long time thereafter. Accordingly, there is now strong demand for fuel efficient oils that retain friction reducing performance even after the lubricating oils have deteriorated due to long-term use.
Of the organo-molybdenum compounds having excellent fuel saving effects, sulfurized oxymolybdenum dialkyldithiocarbamate have drawn special attention. These compounds have long been known as lubricants. For example, Japanese Patent Publication No. 53-31646 describes the use of an sulfurized oxymolybdenum dialkyldithiocarbamate as a lubricant, which compound contains alkyl groups each having 1 to 24 carbon atoms and sulfur and oxygen atoms in specific proportions. Separately, Japanese Patent Publication No. 6-47675 discloses an sulfurized oxymolybdenum dialkyldithiocarbamate where alkyl groups are asymmetric, which is an example of an sulfurized oxymolybdenum dialkyldithiocarbamate having improved solubility in base oils.
Japanese Patent Application Laid-open No. 8-176779 discloses that the aforementioned alkyl-group-asymmetric sulfurized oxymolybdenum dialkyldithiocarbamate has superior solubility also in high viscosity index oil (high VI oil) in which lubricating oil additives have low solubility.
Recent studies, however, have revealed that such alkyl-group-asymmetric oxymolybdenum dialkyldithiocarbamate sulfides are somewhat poor in so-called long drain properties, properties that exhibit superior friction reducing effect even after deterioration of the lubricating oils, although they are superior in base oil solubility.
Accordingly, after intensive investigation, the present inventors have developed a lubricating composition which is superior both in additive solubility in lubricating bases and in long drain properties by the combined use of specific organo-molybdenum compounds and a specific antioxidant.